1. Field of the Invention
The present invention relates to 4′-C-substituted-2-haloadenosine derivatives and use thereof as a medicine, in particular a medicine which is useful for the treatment of acquired immunodeficiency syndrome (AIDS).
2. Background Art
The clinical setting for AIDS has been dramatically changed by a multi-drug combination therapy, which is sometimes called highly active antiretroviral therapy, or HAART. In HAART, nucleoside reverse transcriptase inhibitors (NRTIs) such as zidovudine (AZT), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), and lamivudine (3TC), and protease inhibitors (PIs) are employed in combination.
Although HAART has drastically decreased the number of deaths caused by AIDS, there has emerged a multi-drug resistant HIV-1 (human immunodeficiency virus-1) mutant exhibiting cross-resistance to various drugs. For example, in the early 1990s patients infected with an HIV exhibiting resistance to both AZT and 3TC were very rare, whereas in 1995-1996 the percentage of AIDS patients infected with such an HIV became as high as 42%.
Ohrui, et al. have synthesized 2′-deoxy-4′-C-ethynyl nucleosides and assayed the anti-HIV activity thereof, and as a result, have found that a 2′-deoxy-4′-C-ethynyl nucleoside having a specific structure exhibits potent anti-HIV activity equal to or higher than that of AZT, and has effective antiviral activity against a multi-drug-resistant viral strain exhibiting resistance to various anti-HIV drugs such as AZT, ddI, ddC, d4T, and 3TC. (See, for example, Nucleic Acids Symp. Ser., January 2000, (44): 105-6; J. Med. Chem., November 2000, 43(23): 4516-25; Curr. Drug Targets Infect. Disord, May 2001, 1(1): 1-10; Antimicrob. Agents Chemother., May 2001, 45: 1539-1546; Nucleosides Nucleotides Nucleic Acids, May 2003, 22(5-8): 887-9; WO 00/69876; WO 00/69877; and WO 03/68796.)
The present inventors have evaluated in vitro toxicity of 4′-C-ethynyl purine nucleoside derivatives and 4′-C-cyano purine nucleoside derivatives, which, among a variety of 4′-C-substituted nucleosides, exhibit particularly potent anti-HIV activity. As a result, the present inventors have found that: (1) 2,6-diaminopurine derivatives and guanine derivatives, which exhibit the most potent anti-HIV activity, exhibit toxicity in vitro and in vivo; and (2) adenine derivatives, which exhibit less toxicity, are readily converted into hypoxanthine derivatives in blood by adenosine deaminase, thereby weakening the anti-HIV activity of the derivatives.
In order to attain further enhancement of selectivity index; i.e., (concentration at which cytotoxicity is obtained)/(concentration at which anti-HIV activity is obtained) and to provide resistance to inactivation by adenosine deaminase, the present inventors have synthesized a variety of derivatives through chemical modification of 4′-C-substituted-2′-deoxyadenosine (a lead compound), which, among various 4′-C-substituted purine nucleosides, exhibits potent anti-HIV activity and less toxicity.
As has been known, when a halogen atom, which exhibits electron attraction, is introduced to the 2-position of the base moiety of an adenosine derivative, the resultant derivative exhibits a certain level of resistance to inactivation by adenosine deaminase (Chem. Pharm. Bull., 42(1994), p 1688; J. Med. Chem., 39(1996), p 3847). However, whether or not selectivity index can be improved through introduction of a halogen atom has remained unknown.
Only one literature discloses that introduction of an ethynyl group to the 4′-position of d4T (stavudine: 2′,3′-didehydro-3′-deoxythymidine) enhances the selectivity index of d4T (Bioorg. Med. Chem. Lett., November 2003, 13(21): 3775-7). However, effects similar to those of d4T are not expected to be obtained in an adenosine derivative, which is a purine nucleoside, whose basic skeleton differs considerably from that of d4T, and therefore, this literature does not provide useful information for the present inventors' purposes.